Car bending on launch

[Dusty Bradford]

Have you notched the frame? I have seen this happen in lots of cars with notched frames. If you don't have a fuel cell then there is no weight on your frame behind the axle. Really all the frame does is hold up your rear bumper. The first problem is your cage is not tied to the frame and when you leave the springs and shocks are pushing upwards on the frame. If your frame is not strong enough then it is also flexing upwards causing the rear section of the body to crease as if you were jacking the car up using the rear bumper. A cage must be tied to the frame for real benifits, it's not just about protecting your head but also about strength and tieing things together. The best location to tie the bar in is not to the rear most section of frame (this will help some) but instead tie it into your suspension mounting points. When the cage is tied to the frame and then the rear suspension mount points, there should be no flex.

 

[GO FAST]

The frame is notched, that could be the main cause of the problem. Thanks to all who had constructive replies, and suggestions.

 

[Nick Micale]

Let me relate my experience over a few years with the race car twisting and lifting upon launch. First, I certainly agree the rear bars MUST be attached to the frame, not the body.

The initial items we did were back seat braces and frame stiffening at the roll bar mounts. Like boxing the channel frame and reinforcing the lower control arm mounts.

Car was still twisting enough to crack the windshield. We then replaced all body bushings and welded an X member in the main roll bar hoop. Also removed the rubber mounts on the trans crossmember with alum pieces. This cured all the frame/body twisting.

When the frame was notched/mini-tubbed, a bar/pipe was welded across the frame rails.

With more power, the car was stiff but not twisting, however lifting like crazy - see pic on home page. This was solved by installing the HR Parts anti-roll bar. Just for info, to see how my car now launches, check the videos on my web site how straight and level it goes. Both videos show 9 sec., 1.3 60's.

You may not need to do all the things I did, but some, like the anti-roll bar, are a must-do.

 

[wnarunm]

I have personally seen this on many cars as well, many of which were notched.... But I think the fact that the cars that are denting in the quarter in this area are fast enough that most would have a frame notch anyway. What I HAVE noticed and completely agree with is that most of these cars have the style of cage that keeps the rear seat and the two down bars are attached to the upper control arm mounting points. That said, the rear frame does not have enough support to completely counter the force of the axle twisting under it. The frame of the car where the main cage rests has plenty of support and thats why many cars its actually hard to see a twist in the launch except for this crease. And thats forming becuase the frame's pulling the quarter panel with it (actually noticed it more on a friends car with stiffer body bushings) The best design's would include a down bar that attaches at the upper shock points and then supports bars or an X-brace tied into the lower control arms. Much like the Art Morrison setup.

No rear seat tho..